The present invention relates to medical ultrasonic imaging and in particular to such systems which utilise a catheter insertable into a patient and carrying an ultrasonic transducer array at or near its distal end.
This invention relates to the use of catheters in medical practice, and in particular to using catheters for diagnosis and treatment in the heart, either in the chambers of the heart, or in the coronary arteries. In many procedures it is valuable to have information on the location of the catheter tip within the body. One example is in intra-vascular ultrasound imaging (IVUS) where by pulling back the catheter on a trajectory defined by a guide-wire the two-dimensional images can be stacked to give a three dimensional representation of the inside of the artery. Our UK patent 2,246,632 discloses such a system. In current methods the trajectory is assumed to be a straight line in the absence of other information, and so the representation may not faithfully represent the true three dimensional (3-D) shape of the vessel. Another example of such a procedure is electro-physiology, where the electrical signals of the heart are measured at a number of different positions around the heart wall, and the location of each measurement is important.
There are a number of techniques for position measurement, usually relying on external radiation. The position of the tip on two bi-plane X-rays has been used to calculate the tip position in three dimensions (3-D reconstruction from biplane-angiography by C. R. Moll IBM internal report UKSC124, September 1984). Another technique uses an ultrasound transmitter located in the catheter and external ultrasound imaging, (Bryer et.al. Medical and Biological Engineering and Computing, 268-271, 1985). Many techniques use low frequency magnetic fields to induce a signal in a coil mounted in the catheter, the amplitude of the signal being dependent on the position and orientation of the coil relative to the applied external magnetic field, examples of this method being described in U.S. Pat. No. 4,017,858, U.S. Pat. No. 5,042,486 and U.S. Pat. No. 4,045,881. A further example of this is PCT/US95/01103, where three transmit planar coils are placed on the patient bed, and three orthogonal receive coils placed in the catheter. By exciting the transmit coils at different frequencies, and appropriate analysis, the three spatial co-ordinates and three orientation angles of the catheter tip can be calculated.
There are a number of devices that attempt to measure or control the linear distance that a catheter is inserted into the patient. One example of a device that controls the catheter position by using the motor drive translation of the catheter in the body is given in WO94/00052, and an example of a device which measures the position is given in WO93/20876. These devices have the significant disadvantage that they have to be separated from the sterile field by a sterile drape.